RELATIVE SENSITIVENESS OF Fish Empryos' 3819 
of the Fundulus embryo to rapid and great variations in the 
concentration of the sea-water must rest (at least partly 
[1903 ]) upon properties of the germ-plasm. 
I will not try to say, however, what condition determines 
that the sensitiveness to loss of water is much greater during 
the process of cleavage than during the formation of the 
embryo. 
III. CONCLUDING REMARKS 
The experiments which have just been detailed were made 
with a view to obtaining further data for a theory of embry- 
onal organization, or—as this is ordinarily termed—for a 
theory of heredity. It seems to me that some of these 
theories—for example, Weissmann’stheory of determinants — 
assume more for the germ-plasm than it contains. According 
to this theory, things are already definitely determined in 
the germ-plasm which, to my mind, are functions of circum- 
stances that first make their appearance in much later stages 
of development. To give an example: According to the 
theory of determinants, one would have to imagine that the 
marking of the yolk-sac of Fundulus is already prearranged 
by the spatial grouping of the determinants in the germ- 
plasm which are responsible for the marking, while I found 
that the marking is produced by the protoplasm of the chro- 
matophores being compelled through its ‘‘chemotropism”’ to 
spread over the surface of the blood-vessels. The formation 
of blood-vessels, as well as the formation of pigment-cells, 
may perhaps be traced back to the original germ, but the 
spatial arrangement of the pigment-cell is, as we have seen, 
the effect of a stimulus which the fully developed vessels, or 
rather the blood which they contain, exercises upon the fully 
developed chromatophores. These facts led me to the idea 
that the chemical circumstances determining organization 
do not all exist ready-formed in the germ-plasm, but arise 
gradually in the different stages of development. The 
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